1. Field of the Invention
This invention relates to intersecting optical waveguides with reduced transmission loss. In high-density integrated optical circuits, waveguide crossings are inevitable. One characteristic that results from intersecting waveguide crossings is an intersecting region where light entering is no longer bound by the waveguide geometry. Light in an unbound intersecting region, analogous to a freely expanding wave without boundaries, freely expands and its phase front becomes parabolically curved to the point that the other side of the intersection can no longer collect the entire field of the expanded mode. Transmission loss results from this diffraction of the optical field, and is one of two known deleterious effects that commonly results from waveguide crossings.
Another deleterious effect that commonly results from waveguide crossings is crosstalk. Crosstalk occurs when one field in the first waveguide interferes with another field in the second waveguide. Crosstalk is eliminated when the intersecting region is prevented, by symmetry from decaying into the crossing waveguide, creating a one-dimensional tunneling effect. This effect is minimized here by the use of perpendicular intersections in this disclosure.
2. Description of Related Art
Ideal waveguide crossing design in conventional devices has been a matter of trial and error. The two barriers in creating a prefect waveguide crossing are transmission loss and crosstalk between the waveguides. High losses associated with bends in conventional waveguides, along with the resulting transmission loss, as seen in untapered perpendicular intersections, has forced conventional designers to create shallow-angle crossings that make it even more difficult to achieve low crosstalk. Crosstalk increases when two optical fields spend greater time together as is the case when waveguides intersect at shallow angles.
One such disclosure is described in U.S. Pat. No. 4,961,619, which modifies prior art waveguide crossings having a predetermined angle of intersection by decreasing the dimensions of the waveguides transverse to the direction of propagation, but in the plane of the intersecting waveguides, as the waveguides approach the region of intersection. The disclosure focuses on maintaining the shallow angle of intersection and even reducing the angle below that which is commonly allowed with “unmodified” waveguides.
U.S. Pat. No. 6,198,860 provides for a photonic crystal resonator system, or resonator system by itself, at the intersection of two waveguides. The photonic crystals are made up of materials that restrict the propagation of light to certain frequency ranges.
The high losses associated with bends in conventional waveguides that has forced conventional designers to create shallow-angle crossings is described in U.S. Pat. No. 6,198,860, which specifies the advantage of using photonic crystals. Another alternative to photonic crystals is also discussed in the article of B. E. Little and S. T Chu entitled, “Towards Very Large Scale Integrated Photonics”, Optics and Photonics News, November 2000. The article explains the use of a Manhattan grid as opposed to photonic crystals.
U.S. Pat. No. 5,157,756 describes a waveguide intersection containing an island region analogous to the intersecting region, but also containing a peripheral region having a refractive index lower than that of the waveguides. The invention also claims a predetermined angle of intersection for the waveguides similar to that of U.S. Pat. No. 4,961,619, discussed above.